This invention relates to rotary die cutters for cutting blanks from paperboard and other web and sheet materials, and more specifically to covers for the anvil rolls in such die cutters.
It is the conventional practice on such rotary die cutters to utilize an elastomeric cover on the anvil roll as the cutting surface against which the dies on the die roll cut the feed stock. The elastomeric material most often used is polyurethane, typically of a hardness in the range of Shore A 80 to 95.
Polyurethane anvil roll covers are commonly provided in two general forms. One is a blanket having a sheet steel backing which wraps around the anvil roll and has its ends latched together and to the roll, an example of such blankets being disclosed in the co-owned O'Connor et al U.S. Pat. No. 4,848,204 of 1989. The other common form comprises a pair of semi-circular parts which wrap around supporting structure on the anvil roll that holds them in place, examples being disclosed in Sauer U.S. Pat. No. 3,274,873 of 1966 and Kirkpatrick U.S. Pat. No. 4,982,639 of 1991.
Blankets of both of these forms or types have been found satisfactory in combination with die rolls on which are mounted wood backed steel rule dies including blades having serrated cutting edges for most corrugated paperboard die cutting applications, but not for use where the cut edges of the blanks are required to be as smooth as possible. Serrated cutting edges leave an impression on the cut edge of the finished product which is not acceptable for high quality boxes and a wide variety of other products cut from paperboard, folding carton stock and many other materials.
This combination of blades and anvil roll cover is also not satisfactory for die cutting very thin, pliable or fibrous materials, which tend to be cut with jagged edges, torn or cut with a rolled edge rather than cut smoothly by serrated blades in combination with a soft anvil roll cover. A primary reason for this result is that serrated blade edges require a substantial degree of penetration of the anvil roll cover, and this requirement leads to tearing of the feed stock, especially with thin stretchable plastic film and the like. This problem is prevalent in the cutting of fiber-reinforced webs and non-woven fabrics, with which fibers tend to be embedded in the soft cover rather than cut, so that they dangle from the cut edge and must be separately trimmed.
One approach to this problem has been to use straight edge (non-serrated) blades on the die roll of a rotary die cutter, but this approach also has not thus far proved to be satisfactory. When such blades are used with an anvil roll having a relatively soft cover such as is used with serrated blades, thin, elastic, fibrous or otherwise flexible stock tends to be either not cut or torn rather than cut cleanly.
Attempts have also been made to use straight edge blades in combination with a metal anvil roll, but cleanly cut edges can then be obtained only when the blade edges just meet the surface of the anvil roll, and this requires very precise relative adjustment of the two rolls. Some flexibility in this respect can be obtained by resilient mountings for the blades in the die roll, but this involves complex mechanical means as well as undesirable metal-to-metal contact between the blade edges and the anvil roll which limits the effective life of the blades.
In an effort to overcome or eliminate these problems with past practices, the present inventors have experimented with die cutter blankets like those shown in their above patent but consisting of a pair of super-imposed polyurethane layers and a metal backing, with the outer polyurethane layer being much harder than the inner layer. That approach, however, proved to be unsatisfactory for two reasons.
In the first place, it is very difficult to maintain concentricity of the outer surface of the composite blanket, in part because it is difficult to wrap the sheet steel inner layer of the blanket into uniform contact with the anvil roll. Also, the latching structure at the ends of the blanket created similar difficulties as well as uneven thickness of the polyurethane.
The other reason for the unsatisfactory performance of a dual layer wrap-around blanket was that there was necessarily a junction line between the two ends of the blanket wrapped around the anvil roll, and whenever a blade on the die roll cut across that line, this caused a rough spot on the cut edge of the stock. For similar reasons, the multi-piece covers shown in the above Sauer and Kirkpatrick patents do not produce smooth edges when used with die rolls equipped with non-serrated cutting blades.